This invention relates to screening of slurries to remove objectionable impurities, and more particularly to pressurized pulp screens for removing debris from pulp slurries and paper making stock.
The production of paper making fibers by any known pulping method is incomplete in that certain portions of the wood are not broken down into individual fibers. Such wood portions, which include debris such as shives, bark, slivers, and chop, as well as foreign particles such as sand, grit, and pieces of metal and plastic, must be separated from the paper making fibers before the manufacture of paper can begin. Presently, paper making stocks made from secondary fibers (waste paper stocks) contain much greater amounts of foamed and other light weight plastic particles having specific gravities less than water than have been present in the past.
Removal of these objectionable particles has heretofore been accomplished both by centrifugal cleaning to separate high specific gravity particles from good paper making fibers and by screening to separate large surface area particles from good paper making fibers. In recent years, pressurized screens have become the most common type of screening apparatus in use.
These screening devices generally have a pressurized housing with a cylindrical screen plate dividing the housing into an inner chamber and an outer annular chamber and includes a rotor in the proximity of the screenplate. The slurry to be screened is introduced into the inner chamber and has both a rotary and axial velocity imparted to it by the rotor. The cylindrical plate has slots or holes through which the desirable or accepts fibers pass while the undesirable or rejects particles are retained by the cylindrical screen plate. The accepts and rejects streams are then separately removed from the screening apparatus. Examples of such screening apparatus include Cannon et al. U.S. Pat. No. 2,975,899, and Martindale U.S. Pat. No. 2,835,173.
However, since pressurized screens first appeared commercially, considerable changes have taken place in the industry. The most important of these changes with respect to these screens have been increased capacity requirements and a change in the nature of objectionable particles to be removed. For example, while the early screening devices had cylindrical screenplates on the order of 36 inches in diameter and 12 to 24 inches in height, demands for increased capacity have resulted today in screenplates on the order of 60 inches in diameter and 48 inches in height. However, the relative capacity per unit area of screen surface of these larger screening devices has not been as great as that of the smaller screens.
This loss of capacity may be due, at least in part, to the thickening of the pulp as it moves along the longer screenplates utilized in present devices. This thickened pulp causes a decrease in the hydraulic capacity of a screening device. Moreover, by the time the pulp reaches the base of the screenplate, it may have been accelerated by the rotor mechanism to an extent that the relative velocity differential between the pulp and rotor blades is too small for efficient screening.
Additionally, the standard pressurized screening device in use today was designed with the idea that impurities would be removed through a rejects outlet located at the bottom of the device. Such a design is not efficient where, as with many paper machine furnishes today, considerable amounts of foamed and other lightweight plastic particles are present. Some particles tend to float upwardly when entering the screen housing and are difficult to remove from screening devices having only a single rejects outlet located at the base thereof.
Accordingly, the need exists in this art for a high capacity pressure screen which can efficiently and effectively achieve essentially complete separation of desirable fibers from both heavy and light objectionable particles in a pulp slurry.